If we define user macro like {$VFS.FS.PUSED.MAX.WARN:regex:"/var"} and put in trigger expression - all works fine. But if we decide to clarify it with {$VFS.FS.PUSED.MAX.WARN:regex:"^/var$"} Zabbix Frontend will show next error message it Problems and Event details pages:

By the way, Zabbix Server will process it without any problems.

Also, documentation says that both variants are correct:

https://www.zabbix.com/documentation/current/manual/config/macros/user_macros_context